1. Field of the Technology
The present disclosure relates to methods for processing titanium alloys.
2. Description of the Background of the Technology
Methods for producing titanium and titanium alloys having coarse grain (CG), fine grain (EG), very fine grain (VEG); or ultra fine grain (UFG) microstructure involve the use of multiple reheats and forging steps. Forging steps may include one or more upset forging steps in addition to draw forging on an open die press.
As used herein, when referring to the micro structure of titanium alloys: the term “coarse grain” refers to alpha grain sizes of 400 μm down to greater than about 14 μm; the term “line grain” refers to alpha gram sizes in the range of 14 μm down to greater than 10 μm: the term “very fine grain” refers to alpha grain sizes of 10 μm down to greater than 4.0 μm; and the term “ultrafine grain” refers to alpha grain sizes of 4.0 μm or less.
Known commercial methods of forging titanium and titanium alloys to produce coarse grain or fine grain microstructures employ strain rates of 0.03 s−1 to 0.10 s−1 using multiple reheats and forging steps.
Known methods intended for the manufacture of fine grain, very fine grain, or ultrafine grain microstructures apply a multi-axis forging (MAF) process at an ultra-slow strain rate of 0.001 s−1 or slower (see, for example, G. Sallshohev, et al. Materials Science Forum, Vol. 584-586, pp. 783-788 (2008)). The generic MAF process is described in, for example. C. Desrayaud, et. al, Journal of Materials Processing Technology, 172, pp. 152-156 (2006).
The key to grain refinement in the ultra-slow strain rate MAF process is the ability to continually operate in a regime of dynamic recrystallization that is a result of the ultra-slow strain rates used, i.e., 0.001 s−1 or slower. During dynamic recrystallization, grains simultaneously nucleate, grow, and accumulate dislocations. The generation of dislocations within the newly nucleated grains continually reduces the driving force for grain growth, and grain nucleation is energetically favorable. The ultra-slow strain rate MAF process uses dynamic recrystallization to continually recrystallize grains during the forging process.
Relatively uniform cubes of ultrafine grain Ti-6-4 alloy (UNS R56400) can be produced using the ultra-slow strain rate MAF process, but the cumulative time taken to perform the MAF steps can be excessive in a commercial setting. In addition, conventional large scale, commercially available open die press forging equipment may not have the capability to achieve the ultra-slow strain rates required in such embodiments and, therefore, custom forging equipment may be required for carrying out production-scale ultra-slow strain rate MAF.
Accordingly, it would be advantageous to develop a process for producing titanium alloys having coarse, fine, very fine, or ultrafine grain microstructure that does not require multiple reheats, accommodates higher strain rates, reduces the time necessary for processing, and/or eliminates the need for custom forging equipment.